"Robert Jeffress says, 'You can't be saved by being a Jew,' and 'Mormonism is a heresy from the pit of hell,'" Romney wrote in a tweet. "He's said the same about Islam. Such a religious bigot should not be giving the prayer that opens the United States Embassy in Jerusalem."

Romney is Mormon.

Jeffress denied he was a bigot, but added that he believed Mormonism was "wrong," and said the Southern Baptist Convention had designated it a "cult."

The Rev. Robert Jeffress.Carolyn Kaster

"Mormonism has never been considered a part of historic Christianity. People may disagree with that view, but it's not a view unique to me," he said in an interview with NBC News.

Jeffress bases his beliefs and his general opposition to a two-state solution to the conflict between Israel and the Palestinians on his strict interpretation of the Bible.

"The Bible says this land belongs to the Jewish people — period," he told NBC News in a separate interview in February. "God has pronounced judgment after judgment in the Old Testament to those who would 'divide the land,' end quote, and hand it over to non-Jews."

American evangelicals surged onto the political scene in 1980 by helping to elect President Ronald Reagan. In 2016, around 80 percent of white evangelicals voted for Trump. As evangelicals grew more prominent domestically, their ties to the Israeli political establishment strengthened.

Trump and Vice President Mike Pence have been embraced by Christian Zionists who believe the establishment of the state of Israel is proof of God keeping his promises and a step toward the second coming of Christ.

Jeffress isn't the only conservative evangelical leader to be on hand for Monday's ceremony, which will include around 800 guests. Rev. John Hagee, the founder of influential evangelical Christians United for Israel and a pastor from San Antonio, was also scheduled to deliver a closing blessing at the ceremony.

Many European nations who oppose the decision are expected to skip event surrounding the embassy move.